The present invention relates to an outboard motor capable of achieving an improved noise reduction efficiency with a simple structure.
In general, an outboard motor has an underwater exhaust system. The underwater exhaust system has a structure that, for example, an exhaust tube is disposed in a drive-shaft housing of the outboard motor, the exhaust tube being, with bolts or the like, secured to a member, for example, an exhaust manifold, disposed at a portion upper than the drive-shaft housing in a state of the outboard motor being mounted to a hull. Thus, exhaust gas flowing -from an engine is discharged into the drive-shaft housing.
However, the bolts for securing the exhaust tube are easily corroded because the bolts is frequently immersed into or come in contact with sea water. Thus, there arises a problem in that the bolt is broken during a process for maintaining the outboard motor. What is worse, a multiplicity of parts, including washers, are required in addition to the bolts for securing the exhaust tube, thus causing a great labor to be required to assemble the outboard motor. As a result, the cost for manufacturing and maintaining the outboard motor cannot be reduced.
If the exhaust tube is individually disposed in the drive-shaft housing, a sufficiently large capacity cannot be obtained for the exhaust passage. Thus, a noise reduction effect has been improved by mixing exhaust cooling water for the engine with the exhaust gas. However, there arises a problem in that exhaust cooling water flows backward in the exhaust passage together with the exhaust gas if the pressure of the exhaust gas is raised attributable to a state of the operation of the outboard motor.